Solid lubricants which are embedded in a sliding surface of a sliding member such as bearings, form a thin film on the sliding surface, and exhibit a sliding effect. Therefore, a film-forming capability of the solid lubricants has a large influence on friction coefficient and life of the resultant film. As such solid lubricants, there are known solid lubricants having a layer structure, in particular, those containing graphite as a main component. The graphite exhibits a high resisting force in the direction of load applied thereto, but has a low resisting force in its sliding direction, owing to the layer structure. In addition, the graphite is a soft material and can maintain a good lubrication performance over a broad temperature range of from an ordinary temperature to a high temperature.
However, the solid lubricants containing graphite as a main component tend to be not only insufficient in film-forming capability to some extent, but also unsatisfactory in life of the resultant film when repeatedly exposed to frictional contact. Therefore, the solid lubricants are unsuitable for use in high-load applications.
On the other hand, as the solid lubricants suitably used in high-load applications, there are known such solid lubricants prepared by blending a polytetrafluoroethylene resin, a soft metal such as indium, lead and tin, and a wax with each other. In particular, there have been widely used solid lubricants prepared by blending the ethylene tetrachloride resin, lead and the wax with each other. Such solid lubricants exhibit an extremely low friction coefficient under high-load conditions and an excellent film-forming capability, and a film obtained therefrom has a long life and an excellent self-mending property.
In recent years, lead-free materials tend to be developed in the consideration of avoiding environmental problems. This tendency of the material development has also been present in the field of solid lubricants. However, in the solid lubricants, lead is an important constituent for attaining satisfactory sliding properties. In particular, in the case where the solid lubricant are embedded in pores or grooves formed at the sliding surface of a sliding member such as bearings and used under high-load conditions, lead is important from the standpoint of imparting a good film-forming capability thereto.
For example, in Japanese Patent Application Laid-Open (KOKAI) No. 55-108427(1980), as a lead-free sliding member, there is described the sliding member produced by molding a resin containing an adduct of melamine and isocyanuric acid. However, in the case where the resin composition for the sliding member containing the adduct of melamine and isocyanuric acid is used as a solid lubricant, the friction coefficient thereof tends to be insufficient under high-load conditions. Therefore, it has been demanded to provide a solid lubricant containing no lead and exhibiting sufficient sliding properties even under high-load conditions.